usnic: fix uninitialized param to accept(2)
usnic: fix endpoint destruction on the trunk

Fixes an assertion failure in --enable-debug builds and SEGVs in normal
builds.

I'm not 100% sure I like this model, but it at least seems to be
consistent.  Some variation on this scheme will need to be adapted to
the trunk, where usnic_del_procs() is called by the PML instead of
internally in usnic_finalize().

A related bug (but with different mechanics) is #4832.